Finished EmitCargoOnSell

The broken Lint check is still a problem after all those years. This also does not work as suggested in the empty trait. During selling it destroys the cargo so you can't just queue an Unload activity. You can't even enumerate through all passengers without hitting crashes. However, unloading just the first passenger is good enough for Red Alert's single-manned pillboxes.
Done. This solves all the YAML dependency problems. This may indeed be a good solution for https://github.com/OpenRA/OpenRA/issues/4693 and https://github.com/OpenRA/OpenRA/issues/3768, too. But not here and now.